1. Field of the Invention
The present invention relates to a composite filter media for removal of particles from air and gas streams, and specifically refers to a filter that is cleanable and durable in hot environments. The invention also particularly relates to the use of the filter media in filtration units intended to remove particles from a gas stream entering gas turbines. The filter media of the invention may also be used in filtration units utilized in a variety of other applications, such as internal combustion engines, gas compressors, HVAC systems, electronic cabinets cooling, industrial gas cleaning equipment, and the like.
2. Background
The removal of particulates from a gas stream has long been a practice in a variety of industrial fields. Conventional means for filtering particulates and the like from gas streams include, but are not limited to, filter bags, filter tubes, filter panels and filter cartridges. For convenience herein, the term “filter element” will be used to refer collectively to these types of filtration means.
Muller (U.S. Pat. No. 7,501,003) teaches a composite filter media comprising an electrostatically-charged melt blown layer and an expanded polytetrafluoroethylene membrane layer. The media has utility in filtering ambient air because of its high filter efficiency, long lifetime, and ability to resist water and salt penetration. Filter elements made with composite filter media can remove the submicron particles in air that cause corrosion and fouling of gas turbines. Filter elements comprising composite filter media are available from W.L. Gore & Associates, Inc. (Elkton, Md.).
Filters are often used in extreme temperature environments, which can range from extremely cold in arctic climate to extremely hot in desert environments. Consequently, the filter media and the materials of their construction must withstand wide temperature ranges while maintaining optimal functionality. Polypropylene, although commonly used in filter media, may be susceptible to thermal degradation due to oxidation. Unstabilized polypropylene can begin to decompose almost immediately after formation. At elevated temperatures and in the presence of air, polypropylene may disintegrate to an oxidized powder. Melt blown polypropylene webs are especially sensitive to thermal degradation due to their fine fibers and high surface area. When the polypropylene material of the web is degraded sufficiently, the fine melt blown fibers can crack and break.
In a composite filter media, a melt blown layer made of polypropylene may act as a prefilter to protect the expanded polytetrafluoroethylene membrane from a heavy load of dust particles. Melt blown media made with polypropylene is especially suitable for filtration due to its excellent efficiency, charge stability and low cost. At high temperature, the polypropylene melt blown layer may degrade thermally, causing the filter efficiency of the composite media to drop.
Filters are often cleaned by pulse-jet air or reverse air flow to dislodge the dust particles. This causes mechanical stress on the filters. The repeated mechanical stress and shear on the melt blown fibers may also act as an initiator of oxidation. Over time, the fibers may crack or break. This interferes with the cleaning process and causes the airflow resistance to increase rapidly, thus, reducing the lifetime of the filter.
Antioxidant may be added to polypropylene to improve its resistance to high temperatures. U.S. Pat. No. 4,892,784 Reeves et al. teaches the art of adding antioxidants to polypropylene webs by means of aqueous solution. After the melt blown is made, it is coated with antioxidants which are dissolved in aqueous solution. Many antioxidants are not soluble in water, however, which severely limits the choice of antioxidants. The extra coating step in Reeves, may also add cost and complexity to filter media construction.
U.S. Pat. No. 5,969,026 teaches the addition of low concentrations of antioxidants to the polypropylene resin. The amount of antioxidant is limited of 0.001 to 0.05 percent by weight. Higher concentrations cause process issues such as residues plating on the surface of the equipment. High concentrations of antioxidants can also cause other undesirable effects such as yellowing of the material. More importantly, high amount of antioxidants interferes with the visbreaking process during the melt blown production process. High melt flow is necessary for the melt blown process to produce fine fibers. During visbreaking, the melt flow characteristic of the polypropylene is increased. This is achieved by adding peroxide to the polypropylene resin which acts as free radicals to breakdown the molecular chains and decrease the polymer melt viscosity. Increasing the amount of antioxidant, however, typically interferes with this process because of the reaction between antioxidants and peroxide. To avoid these problems, antioxidant has been added to the polypropylene resin at very low concentrations in previous melt blown webs for filtration applications.
However, the issues involved with thermal degradation of melt blown webs in composite filter media applications remains unresolved.
There thus remains a need in the art for filter media which maintain filtration efficiency and cleanability for an extended period of time at elevated temperatures of filtration operation.